In integrated circuit fabrication, it is often desirable to plate metal onto the surface of a wafer. The plated metal provides assembly sites on the wafer with a “bump” pad for wire bonding. Plating of wafers is done at a known plating hood in which Cu, Ni, or Pd can be used to plate wafers. Within the hood, the wafer is held by a holder during the plating process. The holder assists in the plating process by supporting the wafer and providing metal pins in contact with a copper seed metal surface of the wafer such that a current is applied to the wafer via the metal pins during the plating process.
The older style known plating holder 100, which is being replaced, is depicted in FIGS. 1A and 1B. Two wafers 250 exhibiting two types of exemplary damage from the known wafer holder 100 are depicted in FIGS. 2A and 2B. The known plating holder 100 uses two separate and disconnected pieces, including a base piece 110 and a top piece 120. The base piece 110 includes a wafer supporting surface 112 and a handle 114. A wafer 150 is seated on the wafer supporting surface 112. The top piece of the holder 100 includes four contact pins 122 protruding from a wafer facing surface. Wafers 150 and 250 include four contact points or openings through the patterned photo resist to the seed metal at points 152 (in FIG. 1A) and 254 (in FIG. 2A). The four metal contact pins 122 of the holder 100 engage with the four contact points 152 and 254 of the wafer 150 or 250, respectively. In addition, wiring to a power supply 124 is connected to the top piece 120 of the holder 100 for supplying current to the four holder metal contact pins 122. The base piece 110 and the top piece 120 are clamped together with four clamps 140 around a circumference of the holder 100. The four clamps 140 correspond in location to the four metal contact pins 122.
Metal electroplating is accomplished by delivering current to the metallized contact points 152 and 254 of a wafer surface (seed metal) through the four contact pins 122, acting as conductors pressed against the contact points of the wafer. In order to make the necessary contact, the metal pins 122 of the holder top piece 120 must be aligned with the four contact points 152 of the wafer after the wafer has been placed in the base piece 112 of the holder 100. This wafer opening/holder pin alignment must be done while clamping the four clamps 140 of the holder top piece 120 with the base piece 110 of the holder 100. The top piece 120 is clamped to the bottom piece 110 each time a wafer 150 is loaded in the holder 100. An operator must visually align the contact pins 122 of the top piece 120 with the contact points 152 of the wafer 150 held in the bottom piece 112. When trying to clamp the holder top piece 120 to the bottom piece 110, visual line of sight is impaired, causing holder pin 122 to wafer 150 contact point 152 alignment challenges, which can lead to wafer damage and wafer scrap. If sufficiently good contact is not established between the holder contact pins 122 and the wafer contact points 152, the wafer 150 must be scrapped because a power supply cannot reach the correct voltage to deliver the required set-point current to the wafer. This type of scrap is referred to as high voltage or reverse plating scrap because arcing occurs between the wafer seed metal openings 152 and the holder metal pins 122, resulting in wafer seed metal damage. When this high-voltage arcing, or reverse voltage bias occurs, the seed metal is plated to the holder's metal pins 122 and not the wafer.
An additional type of scrap occurs because the pressure between the holder contact pins 122 and the contact points 152 of the wafer 150 can result in wafer breakage.
FIGS. 2A and 2B illustrate high voltage scrap wafers and broken wafers 250, respectively. The high voltage scrap shown in FIG. 2A resulted from poor pin placement and/or alignment between a known four-pin holder and the four wafer 250 contact points 254. Pins were seated on the photo resist rather than the metallized contact point (wafer seed metal) or opening of the wafer, resulting in the high voltage scrap.
FIG. 2B depicts a broken wafer resulting from excessive stress transfer over the short distance between adjacent pins on the four-pin holder. The four-pin holder excessively stresses the wafer 250 due to pressure from the four contact pins (not shown, but see FIGS. 1A and 1B) on the contact points 254 of the wafer. The holder induced stress can cause breaks between adjacent contact points 254 on the wafer as depicted in FIG. 2B. When the contact pins of the holder press on the wafer contact points, they tend to break the wafer in a straight line 256 from one wafer contact point 254 to the adjacent wafer contact point.
High voltage scrap is caused by the holder pins not being aligned with the wafer contact points 254 on the wafer 250. When the holder metal pins are not seated directly on the wafer contact points, good contact to the wafer is lost for current delivery to the wafer. Proper contact is lost when the misaligned holder pins make contact with the wafer contact points 254. When this occurs, there will be photo resist between the metal pins 122 of the holder 100 top piece 120 and the wafer contact points 254, instead of making contact with the bare copper seed metal. This photo resist acts as an insulator and forces the power supply to drive up the voltage to deliver the proper current to the wafer. During the process of driving up the voltage, in an attempt to deliver the proper current through the photo resist, there occurs what is called High Voltage scrap.
In the art, and in order to avoid generating high voltage scrap and wafer breakage, instead of reducing the number of pin contacts with the wafer, holder manufacturers are increasing the number of pin contacts with the wafer. As an example, E&G Partners has a tool that uses 50 pin contacts. Semitool uses a patented holder that makes continuous contact with the entire circumference of the wafer.
There continues to be a need for improved plating that avoids wafer breakage and wafer scrap.